Deformation and cracking phenomena in cold sprayed 6061 Al alloy powders with nanoscale aluminum oxide films

Abstract

This paper presents deformation and cracking phenomena associated with the cold spray of nanoscale surface oxide layers associated with the cold spray of 6061 Al alloy powders. The structure of the top surface oxide film was revealed via Focused Ion Beam (FIB), Transmission Electron Microscopy (TEM), and X-ray Photoelectron Spectroscopy (XPS). The observations show the oxide layer characteristics most likely matches crystalline γ-Al2O3. Phenomena associated with surface contacts, contact-induced elastic-plastic deformation, heating, and cracking are then studied using a combination of analytical models, Finite Element Analysis (FEA) and Molecular Dynamics (MD) simulations. MD simulations are also used to obtain estimates of oxide film young's moduli, strains to failure, and the fracture energies of oxide films. The results from the MD simulations and microscopy observations are incorporated into a bi-linear Johnson- Cook finite element simulations of cold spray contact-induced deformation and cracking due to the impact of aluminum powder particles with nanoscale oxide layers and similar substrates with nanoscale oxide layers. The powder impacts are shown to result in localized splat deformation and heating, along with the cracking of the oxide layers that expose fresh metallic surfaces to high temperature surface contacts (at temperatures above the recrystallization temperature for aluminum/6061 Al) that can give rise to bonding and mechanical interlocking. The implications of the results are then discussed for the design of cold spray processes for the fabrication and repair of 6061 Al structures.